Bug 1704750 - Use simpler check for triangle winding. r=aosmond
Bug 1704750 - Use simpler check for triangle winding. r=aosmond

The original triangle winding check was designed to be run inside the span
rasterizer every time the edges changed. However, at present, this only ever
gets used once at the very top of the triangle to determine winding and then
assumes winding is constant thereafter.

This updates the check to work on these assumptions to be simpler and less
fragile where numerical precision is concerned. If we can assume the candidate
edges both start on the same row, then the check boils down to seeing if they
share the same starting point and doing a cross-product.

Differential Revision: https://phabricator.services.mozilla.com/D113900

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/097daf3a851d841f247ef0d20e2e0d872bfcb754

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender